I have been in exactly your position. Long story short: group of women wanted to bring AEPhi to campus, Panhel denied the request for expansion, group formed local sorority instead... two years later, group re-approached Panhel about AEPhi, Panhel said yes, group met with AEPhi, match was made.
If you want to colonize a chapter of this NPC sorority, you will need (a) a group of women and (b) the blessing of Panhel (if there are other NPC sororities on your campus - I don't recall if there are) to get started. Then, Panhel (or the Greek Life office) will inform NPC that your school is open for expansion, and that there is a group of women with a preference for XYZ. Any NPC sorority not already represented can then express interest in colonizing. If XYZ is interested, your group can meet with them, and if both parties agree, you can become an XYZ colony. If not, you have the option of meeting with any NPC sororities that are interested. You may have to go through a colonization process where every women is interviewed, or the NPC sorority may take your group lock, stock, and barrel.
And as PsychTau pointed out, there is the option of alumna initiation for many orgs.
Good luck with whatever you decide.
